cli: Extract CLI configuration into separate crate

This patch adds the `tree-sitter-config` crate, which manages
tree-sitter's configuration file.  This new setup allows different
components to define their own serializable configuration types, instead
of having to create a single monolithic configuration type.  But the
configuration itself is still stored in a single JSON file.

Before, the default location for the configuration file was
`~/.tree-sitter/config.json`.  This patch updates the default location
to follow the XDG Base Directory spec (or other relevant platform-
specific spec).  So on Linux, for instance, the new default location is
`~/.config/tree-sitter/config.json`.  We will look in the new location
_first_, and fall back on reading from the legacy location if we can't
find anything.

cli/src/highlight.rs

@@ -4,6 +4,7 @@ use anyhow::Result;
 use lazy_static::lazy_static;
 use serde::ser::SerializeMap;
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
+use serde_derive::{Deserialize, Serialize};
 use serde_json::{json, Value};
 use std::collections::HashMap;
 use std::fmt::Write;
@@ -56,6 +57,12 @@ pub struct Theme {
     pub highlight_names: Vec<String>,
 }
 
+#[derive(Default, Deserialize, Serialize)]
+pub struct ThemeConfig {
+    #[serde(default)]
+    pub theme: Theme,
+}
+
 impl Theme {
     pub fn load(path: &path::Path) -> io::Result<Self> {
         let json = fs::read_to_string(path)?;